Chris Evans is squirming. It’s a brisk October morning on the London set of Captain America: The First Avenger, and the 29-year-old actor is decked head to toe in the red, white, and blue threads of the titular Marvel Comics super-soldier, hanging from wires as a massive fan hammers him with wind. On ”action,” Evans drops onto a black train car. At ”cut,” the star hops up with a wince and wiggles his caboose. It seems the wire harness hidden in his trousers isn’t being kind to him. ”There was a lot of business getting choked down there,” Evans later says with a smile, during a break in filming. His wirework appeared flawless, but a stumble on the first take has left him feeling self-conscious. ”I almost fell off the train! That would have been a disaster,” says Evans. ”Did everything else look good?”

He’s being a bit hard on himself, although Marvel Studios is surely grateful for Evans’ dedication to getting it right. Captain America — due July 22, about three months after the company’s other major 2011 release, Thor — isn’t just another new-model masked marvel from the superhero-movie factory that gave us Iron Man, X-Men, and Spider-Man. According to Marvel Studios chief Kevin Feige, Captain America is ”the last jewel in the Marvel crown that hasn’t gotten its own movie franchise.” The estimated $140 million action-adventure must also creatively pave the way for the company’s most ambitious opus yet: The Avengers, a team-up of Marvel’s various movie icons slated for 2012. To paraphrase the Avengers’ fabled motto: Corporate Synergy Assemble!

But first, there’s The First Avenger. Created in 1941 by the fabled comic-book creative team of Joe Simon and Jack Kirby, Captain America was a military man with a shield made of indestructible ”vibranium” who began his comic-book career battling Axis-aligned supervillains — and, on the cover of Cap’s first comic, Hitler himself. In 1964, Marvel Comics brought the character into modern times using a frozen-in-ice conceit, and since then, he has been at the forefront of the Marvel universe, compelling to readers for his Rip Van Winkle angst and provocative to comic-book writers prone to use him as a prism to explore changing attitudes about America and patriotism.

The movie hews closely to Captain America’s WWII-era origins. The year is 1942, and Steve Rogers is a scrawny young man of sterling character burning to fight Nazis but unable to because he’s been deemed physically unfit. His fate — and his physique — is radically transformed when he signs up for Project: Rebirth, a secret military operation that turns wimps into studs using drugs and assorted sci-fi hoo-ha. There’s a love interest (Major Peggy Carter, played by Hayley Atwell), a sidekick (Bucky Barnes, played by Sebastian Stan), and the villain: the Red Skull (Hugo Weaving), Hitler’s treacherous head of advanced weaponry. The Red Skull’s plan for world domination involves a magical object known as the Tesseract (comic fans know it better as the Cosmic Cube), a criminal organization known as HYDRA, and the bombing of New York City. No, this is not a fact-based period piece. ”It’s very much the Marvel Comics version of World War II,” says co-producer Stephen Broussard.

COMIC-BOOK diehards can expect a movie filled with winks and nods to a broad swath of Marvel lore, but the studio insists that those who don’t know their ”Dum Dum” Dugan from their Arnim Zola won’t be left in the cold. Marvel execs cite Raiders of the Lost Ark as a creative touchstone — which is a big reason Marvel entrusted the production to director Joe Johnston (The Wolfman), who earned an Oscar as a member of the visual-effects team on the Steven Spielberg classic. ”The interesting thing about this character is that he’s an Everyman who in the course of a few minutes becomes a perfect human specimen. That has to create some interesting personal issues,” says Johnston, whose résumé also includes another comic-book-derived WWII-era superhero flick, 1991’s The Rocketeer. ”I saw this as an opportunity to make a superhero movie that felt real, that didn’t have to rely on an overabundance of fantasy elements.”

Playing Captain America may seem like a glorious mission for most actors. But Chris Evans initially wanted no part of the movie — and Marvel initially wanted no part of Chris Evans. The studio tested a number of actors for the role, reportedly including Channing Tatum (G.I. Joe), John Krasinski (The Office), and Ryan Phillippe (Crash). Feige says Evans wasn’t on the original wish list, mostly because he had already served a tour of duty in Marvel’s cinematic army as Johnny Storm, a.k.a. the Human Torch, in Fox’s Fantastic Four movies. But as the studio began broadening its search, Evans emerged as a leading candidate. Feige credits the change of heart to the actor’s performances in several little-seen dramas (including the 2007 sci-fi flick Sunshine) and a strong endorsement from Edgar Wright, who directed Evans in Scott Pilgrim vs. the World. He also met an essential requirement: ”We wanted an American,” says Feige.

Yet Evans says he turned down the role at least three times. He worried that audiences wouldn’t accept him as another superhero. He worried that Marvel’s request for a nine-movie commitment — a Captain America trilogy, an Avengers trilogy, and appearances in three more Marvel movies to be named later — would leave little time for other things, including the chance to pursue his dream of directing. He worried about what would happen to his career if the movie flopped. ”I’ve made some spotty films in the past, and I didn’t want another one on this scale,” says Evans, who most recently appeared in two franchise nonstarters, The Losers and Push. At the same time, he worried about the cost of fame if the film succeeded. (”I remember telling a buddy of mine, ‘If the movie bombs, I’m f—ed. If the movie hits, I’m f—ed!”’) Even after he succumbed to Feige’s repeated pleas to take a meeting at Marvel HQ — and even after leaving that meeting intrigued and moved by Captain America’s character arc and dazzled by Johnston’s plans and designs — Evans was wary, but he really didn’t know why. And then it hit him.

”I was just scared,” he says. ”I realized my whole decision-making process was fear-based, and you never want to make a decision out of fear. And so one weekend, I just said, ‘F— it. Let’s do it.”’ (It helped that Marvel agreed to settle on a six-picture deal.) Feige says he was actually impressed by Evans’ reluctance; he believed it reflected a maturity befitting Steve Rogers. Adds Johnston, ”He has really brought a whole different level to the character that I didn’t know existed — more real, more complicated, more vulnerable.”

Evans began prepping for the role by doing what anyone would do in his situation: He went online and read what the fans thought of his casting. The takeaway: He needed bigger muscles. So he worked with a personal trainer to pump his biceps and tone his abs, all of which are on full display during the Project: Rebirth sequence. (In order to show Steve Rogers as a scrawny runt, Johnston plans to use a combination of clever camera angles, costume trickery, and special effects that will digitally reduce Evans’ body or even put his face on a skinny actor.)

The meatiest issue facing Captain America is the politics. Marvel began the project keenly aware that a movie about a star-spangled supersoldier named Captain America could be a tough sell to a politically divided nation and international audiences with a dim view of America’s current role in world affairs. Indeed, Feige indicates that Captain America: The First Avenger may be released in some foreign territories as just The First Avenger. Still, says Johnston, ”I never wanted to make this movie into something of a flag-waver. We were very careful about that when we were developing the screenplay.” Marvel initially wanted a movie that toggled between past and present, but ultimately opted to set the story exclusively during WWII because the back-and-forth approach didn’t work for an origin story — and because WWII has exactly zero moral ambiguity. Johnston says he also wanted a movie ”about international cooperation.” To that end, the film makes Captain America the leader of a team of elite soldiers from various countries, known in the comic books as the Howling Commandos. ”It’s not about running from [political interpretations] or being afraid of saying anything, but just staying true to what the story has always been,” says Broussard. ”I welcome whatever kind of healthy debate comes from it.”

Even more welcome for Marvel would be a movie that leaves audiences wanting to sign up immediately for The Avengers, which should be well into production under writer-director Joss Whedon (Buffy the Vampire Slayer) by the time Captain America opens in July. Joining Evans in the cast will be Robert Downey Jr. (Iron Man), Samuel L. Jackson (Nick Fury), Scarlett Johansson (Black Widow), Mark Ruffalo (Hulk), Chris Hemsworth (Thor), and Jeremy Renner (Hawkeye). Captain America: The First Avenger will set up the film’s premise — and bring Captain America into the present — with a prologue and epilogue that work the frozen-in-ice angle that Marvel used to revive the Captain America comic-book franchise in 1964. For all his early apprehensions, Evans now has no regrets about committing to a role that could dominate his Hollywood life until his 40th birthday. ”When I first put on the suit, I was absolutely terrified. But once I started working, I could just see this was going to be a good experience. Then I started going, ‘Wow. This is really cool,”’ he says. ”I can’t believe I was almost too chicken to play Captain America.”